Identification and enumeration of analytes in complex sample matrices are used in medical, biological, industrial, and environmental applications. Example analytes include particles such as viruses, bacteria, parasites, and specific cell types, typically found in a complex matrix of confounding substances. Sample preparation methods such as filtration, lysis, homogenization and dilution are often required to enable specific particle identification and enumeration in these complex matrices. Particle identification and enumeration are often based on expensive, laboratory-based measurement devices or instrumentation.
A useful example is the identification and enumeration of CD4+ T-helper lymphocytes (CD4 cells) for monitoring and managing conditions in persons with HIV/AIDS. HIV mediated CD4 cell destruction is the central immunologic feature of HIV infection. Thus, the CD4 count is a critical measurement in initial assessment of infection and disease staging, in monitoring antiretroviral therapy and in managing primary and secondary prophylaxis for opportunistic infections. In fact, quantitative T helper cell counts in the range of 0 to 1000 cells per microliter are a critical indicator for initiating and optimizing antiretroviral treatment and preventing viral drug resistance. Flow cytometry is the current standard-of-care for CD4 cell counting. Unfortunately, flow cytometry is a central lab-based technique; transport, equipment, and operational costs render the technique cost-prohibitive in limited resource settings where HIV prevalence is highest.